Diaryl esters of aromatic dicarboxylic acids have been produced by a number of reactions. Foremost among these is the reaction of aromatic dicarboxylic acids, such as isophthalic and terephthalic acids with an aromatic monohydroxy compound, for example phenol. The basic process has been improved or modified in a number of ways.
By way of illustration, in U.S. Pat. No. 3,413,336 an additional agent, acetic anhydride, is employed in carrying out this reaction which is used with equivalent amounts or a slight excess of a phenol (col. 2, lines 31-32). In col. 3, lines 2-3, it is also taught that it is generally not advisable for reasons of economics, to use excess phenol of more than 25%. The specific examples I-V disclose respectively 2 moles+25%, 2 moles+10%, 2 moles+20% and 2 moles+20%.
In U.S. Pat. No. 4,124,566 it is disclosed that in the most preferred state of the esterification reaction the aromatic monohydroxy compound, e.g., phenol, is present only in a low concentration in the aromatic dicarboxylic acid slurry (col. 6, lines 29-34). In the Examples, the molar ratio of the acid to the phenol is about 1 to 4 and 1 to 6 moles. Furthermore, an aromatic hydrocarbon medium such as ethylbenzene is required in the process.
Neither of the above-mentioned patents, U.S. Pat. No. 3,413,336 or U.S. Pat. No. 4,124,566, refer to or deal with the half ester impurity or its conversion to the diester. In U.S. Pat. No. 3,413,336, col. 3, lines 39-42 it is disclosed that the diaryl esters are obtained in a yield of over 90%. It is taught that they may be easily obtained in any desired degree of purity by simple distillation or recrystallization. In Examples I-IV the diaryl ester is distilled off to give yields of 93%, 90%, and 90.5% respectively.
In U.S. Pat. No. 4,124,566, which is concerned with preparing polyesters, the diaryl ester product obtained in the first step is directly converted into a polymer by reaction with an aromatic dihydroxy compound, such as bisphenol A. There is no mention of a half ester or purification of the diaryl ester product either by crystallization or distillation before it is converted into the polymer.
Witt et al., U.S. Pat. No. 3,694,490 discloses the production of the diphenyl ester of terephthalic and isophthalic acid to the exclusion of the corresponding monophenyl ester, by the continuous removal of water by-product from the reaction zone of a heated solution of reactants. Witt et al. suggest that crystallization of the ester, upon cooling, from a monofunctional phenol provides extremely good purification (col. 4, lines 51-52) but they do not disclose any level of purity for the ester or means for crystallizing other than cooling.
Ranade, U.S. Pat. No. 4,451,664; 4,482,732 and 4,482,733, discloses processes for manufacturing diaryl esters of dicarboxylic acids, such as diphenyl isophthalate (DPI) and diphenyl terephthalate (DPT). In U.S. Pat. No. 4,451,664, the esterification reaction is optimized by continuously removing water and other by-products from the reactor while supplying to the reactor a make-up amount of phenol containing no more than about 100 ppm of water. The make up stream may be obtained by stripping the by-product stream of water and other reaction by-products. In U.S. Pat. No. 4,482,732, water and/or lower alkyl alcohols are distilled from the reaction mixture by varying the reflux ratio during the course of reaction. In U.S. Pat. No. 4,482,733, the esterification reaction is optimized by gradually increasing the temperature during reaction in accordance with the degree of esterification. All of these processes suffer from a drawback in that a significant amount of energy must be expended to obtain a degree of esterification of 95% or higher.
In Japanese Patent Application, No. 60/39067, DPT and DPI are reported to be produced in 100% conversion by adding diaryl esters and aromatic hydrocarbons to the reaction mixture before the esterification reaches 20% completion, followed by refluxing. This process is also deficient in the sense that considerable energy must be expended in the recycling of materials to make the diphenyl ester.
Japanese Patent Application No. 59/36644 describes the purification of DPT by recrystallization from aromatic solvents containing amines with no active hydrogens. It would be more desirable, however, to purify diphenyl esters without using amine components.
In Japanese Patent Application, No. 58/213734, DPT is prepared by transesterification of meta- and para-methylbenzoate with phenyl acetate to give high yield (96%) of DPT. This method is disadvantageous because it uses costly reactants to produce the diphenyl ester, and it is not a simple direct esterification.
Japanese Patent No. 55/100339 describes the preparation of diphenyl esters by reacting a carboxylic acid and a diaryl carbonate at 150-350.degree. C. with stirring to give high yields, e.g., 99.5% of DPT. This transesterification process uses costly reactants to make diphenyl esters and is not preferred for this reason.
In European Patent Applications Nos. 44509 and 44510, Jan. 27, 1982, are disclosed processes for preparation of DPI and DPT. In EP No. 44509, the corresponding acid is reacted with diphenyl carbonate in the presence of catalyst to recover the diphenyl ester. In EP No. 44510, high yields of diesters (90-95%) plus a 5-10% yield of the corresponding monoaryl ester or half ester acid are said to be obtained by reacting the aromatic dicarboxylic acid with large molar excess of phenol and removing the water formed during reaction. Neither of these processes provide for yields of a diaryl ester product that contains less than 1 to 5% of the corresponding intermediate monoaryl ester or half ester acid.
In view of the foregoing, it is unexpected now to discover that diaryl esters of aromatic dicarboxylic acids can be easily produced containing less than 1 to 5% of the corresponding half ester if water is added to the reaction mixture and if the temperature during recovery is carefully controlled. The process of the invention is simple, requires low expenditures of energy, does not involve transesterification, and does not use expensive reactants, such as diaryl carbonate. Phenol or other aryl hydroxy compounds are reactants and water is a by-product in the direct esterification reaction. In distinction to the prior art, also, more water is added herein. This novel method of synthesis also advantageously permits the catalyst and unreacted components to be recycled to the reaction efficiently.